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Chem. Mater. 1999, 11, 2372-2381
Clay-Polyvinylpyridine Nanocomposites K. G. Fournaris, M. A. Karakassides, and D. Petridis* Institute of Materials Science, NCSR “Demokritos”, Ag. Paraskevi, Attikis, 153 10, Athens, Greece
K. Yiannakopoulou Institute of Physical Chemistry, NCSR “Demokritos”, Ag. Paraskevi, Attikis, 153 10, Athens, Greece Received December 22, 1998. Revised Manuscript Received May 13, 1999
Nanocomposites of montmorillonite mineral with poly-4-vinylpyridinium salts (1,2-form) the quaternized ionene polymer (1,6-form) and poly-4-vinylpyridine (neutral form) have been synthesized and characterized. Only one macromolecular sheet of poly-4-vinylpyridinium polyelectrolyte or the quaternized polyelectrolyte enters the interlayer space, irrespective of the amount of the polyelectrolyte used in the intercalation. Exfoliated hybrids are not therefore generated with polycationic polymers. In contrast, partially protonated poly-4vinylpyridine is adsorbed at variety of levels and may induce clay exfoliation. Polymerization of monomeric 4-vinylpyridinium salts in the clay galleries is faster than that of the pure 4-vinylpyridinium salt and results in the formation of the quaternized ionene form independently of the polymerization conditions. Adsorption isotherms for the different forms of poly-4-vinylpyridine reveal that surface saturation coverage increases in the following order: partially protonated poly-4-vinylpyridine > quaternized ionene form > completely protonated poly-4-vinylpyridine. Models explaining the different uptake of the three derivatives and the surface selectivity toward quaternized polycations are proposed. The electrochemical results confirm that intercalative polymerization produces only the quaternized form of the polymer and reveal that protonation of poly-4-vinylpyridine occurs because of the acidity of the clay layers.
Introduction The ability of layered silicates to undergo intercalation by a wide variety of monomers or polymers has been studied for many years, because the resulting intercalates are important in agriculture and industry.1,2 Recently the development of new generations of polymersilicate nanocomposites has gained interest, owing to the new techniques available for carrying out structural and property characterization and the improved mechanical, thermal, optical, and other properties, which hold promise for novel technological applications.3-13 (1) Theng, B. K. G. The chemistry of clay-organic reactions; Adam Hilger: London, 1974. (2) Theng, B. K. G. Formation and properties of clay-polymer complexes; Elsevier: Amsterdam, 1979. (3) Okada, A.; Kawasummi, M.; Kurauchi, T.; Kamigaito, O. Polym. Prep. 1987, 28, 447. (4) Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci. Part A: Polym. Chem. 1993, 31, 2493. (5) Mehrota, V.; Giannelis, E. P. Solid State Ionics. 1992, 51, 115. (6) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694. (7) Messermith, P. B.; Giannelis, E. P. Chem. Mater. 1994, 6, 1719. (8) Vaia, R. A.; Vasudevan, S.; Krawiec, W.; Scanlon, L. G.; Giannelis, E. P. Adv. Mater. 1995, 7, 154. (9) Wong, S.; Vasudevan, S.; Vaia, R. A.; Giannelis, E. P.; Zax, D. J. Am. Chem. Soc. 1995, 117, 7568. (10) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8, 1728. (11) Lan, T.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 2216. (12) Wang, M. S.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 468.
The interaction of layered silicates with polymers leads to two classes of hybrid materials. In the first class, denoted as intercalated hybrids, one or more polymer chains are inserted between the host layers, generating ordered lamella with a repeat distance of few nanometers. In the second, described as delaminated hybrids, silicate layers of 1 nm thickness are exfoliated and dispersed in the polymer matrix.14,15 Both classes of clay-polymer nanocomposites can be obtained either by intercalation of monomers followed by interlayer polymerization or by polymer intercalation from solution, providing that a water soluble polymer is available. Recently, a more general approach based on direct intercalation by a molten polymer has been developed, and its advantages in technological applications have been described.6-8 The melt intercalation method is remarkable in that it can produce both intercalated and delaminated composites with a wide range of polymers from nonpolar polystyrene to weakly polar poly(ethylene terephthalate) to strongly polar nylon.10 Polyelectrolytes are a class of important polymers characterized by a large number of ionizable groups. (13) Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 2144. (14) Giannelis, E. P. Adv. Mater. 1996, 8, 29. (15) Pinnavaia, T. J.; Lan, T.; Wang, Z.; Shi, H.; Kaviratna, P. D. ACS Symp. Ser. 1996, 622, 250.
10.1021/cm981140z CCC: $18.00 © 1999 American Chemical Society Published on Web 08/13/1999
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Scheme 1
Experimental Section
Clays having negatively charged surfaces are expected to interact electrostatically with cationic polyelectrolytes. A typical class of cationic polyelectrolytes is protonated polyvinylpyridines and the corresponding N-alkylated derivatives. Although polyvinylpyridines find various applications,16 their interactions with clay surfaces are not well-studied.17 Protonated poly-4-vinylpyridine is a polyelectrolyte with certain advantages in studies with clays. The first is that the 4-vinylpyridine monomer in the protonated form is expected to easily intercalate into the clay layers by an ion exchange process. The inserted monomer can then undergo polymerization within the spatially constrained interlamellar space of the clay. The bulk polymerization of protonated 4-vinylpyridine (4-VPH+X-) in aqueous solution follows two distinct routes which, according to the monomer concentration and acidity of the medium, yield either the 1,2-polyelectrolyte (I) with the pyridinium units in the side chain or the 1,6-polyelectrolyte (quaternary form) (II) with the pyridinium units in the main chain, as shown in Scheme 1.18 The second is that the neutral poly-4-vinylpyridine is easily protonated to form a water soluble, positively charged polyelectrolyte which is expected to insert into the clay lattice. Third, melt intercalation of neutral poly-4-vinylpyridine is feasible if an acidic clay or suitably modified clay is used as the inorganic partner. These features make polyvinylpyridine an attractive system to study the reactivity of clay surfaces with the monomer or polymer, the latter in solution or in melt. In addition, the intercalative polymerization of the monomer offers the possibility to examine whether the clay surfaces show selectivity for the 1,2- or 1,6-mode of polymerization. In view of these potentialities, we report here the synthesis, characterization, and probable structural models of nanocomposites obtained from the interaction of smectite clays with polyvinylpyridine derivatives and the intercalative polymerization of the 4-vinylpyridinium monomer.
Materials. The clay used in this study was a montmorillonite from the island of Milos, Greece, with a stoichiometry 〈Na0.63K0.07Ca0.11〉[Si7.75Al0.25]{Al3.21Mg0.69Fe3+0.02Fe2+0.03Ti4+0.05}O20(OH)4. The mineral was fractionated to 1,6-polyelectrolyte > 1,2-
polyelectrolyte. The data (dotted line) show that at a bulk polymer concentration of 7 × 10-3 mol, 1 g of clay can adsorb 1.75 × 10-3 mol (326 mg) of 1,2-PVPH+Br-, 2.6 × 10-3 mol (484 mg) of 1,6-PVP+Br-, and 2.85 × 10-3 mol (299 mg) of P-4-VP. We first note that neutral P-4-VP is adsorbed in amounts almost twice those of the structurally similar protonated 1,2-polyelectrolyte. Given that the two polymers have similar dimensions, the data suggest that the aluminosilicate layers are laminated by a bimolecular layer of the P-4-VP macromolecule (Figure 12c). This finding, which is in agreement with the XRD results, implicates that the silicate layers are only weakly pinned by the partially protonated P-4-VP, and therefore the penetration of a second polymer layer of P-4-VP becomes feasible. The rise of
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Figure 13. Schematic representation of the placement of (a) 1,2-polyelectrolyte, (b) 1,6-polyelectrolyte, and (c) poly-4-vinylpyridine into montmorillonite.
the adsorption curve at higher bulk polymer concentrations indicates that more polymer layers can be embedded into the lamellar region. Turning to the different adsorptions observed for the 1,2- and 1,6-polyelectrolytic derivatives we attribute this result to the different chain width of the two polymers. The narrow size of the 1,6-polyelectrolyte, estimated near 5 Å, allows the intergallery accommodation of one more polymer with their counteranions, while the large width of about 10 Å for the 1,2-form inhibits an analogous adjustment. A view illustrating the probable placement of the various polyvinylpyridine derivatives in the clay layers is shown in Figure 13. The sketch based on XRD measurements, adsorption isotherms, and size of the polymers satisfactorily explains the experimental results. Electrochemistry. Poly-4-vinylpyridine and its alkylated derivatives have played an important role in the
development of modified electrodes.25 Anson and coworkers have reported the use of protonated P-4-VP or quaternized PVP for the electrostatic binding of anionic redox active solute and discussed the advantages and limitations of these polyelectrolytic coatings.26 The use of clay-polyvinylpyridine nanocmposites in the modification of electrode surfaces is expected to combine the advantages of both the organic and inorganic partners. We discuss briefly the voltammetric response of electrodes modified with a film from clay-polyvinylpyridine hybrids (Figure 14) with the aim to demonstrate certain basic properties of the nanocomposites. Electrode Modification with a Clay-1,2-PVPH+ Hybrid. The uptake of anionic electroactive solutes from solution will depend on the extent of protonation (25) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 247. (26) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 640.
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of the “free” pyridyl groups in the polymeric backbone and therefore upon the pH of the medium. The voltametric results presented in Figure 14a show clearly that the intensities of the oxidation or reduction currents from the [Fe(CN)6]3-/4- couple depend on the pH of the solution and therefore upon the degree of protonation of the coating. It is important to note that a voltammetric wave of sufficient intensity persists even at pH 7 of the medium, meaning that an adequate number of pyridyl groups in the clay galleries are protonated as a result of the increased acidity of the clay layers. The reduction wave at pH 7 amounts to about 50% of the wave at pH 4. By contrast, a bare graphite electrode exhibits a wave at pH 7 about 1% of the wave at pH 4 (data are not shown). Electrode Modification with a Clay-1,6PVP+ClO4 Hybrid and Interlayer Polymerization. Experiments conducted at pH 4 and 7 for the [Fe(CN)6]3-/4- couple (Figure 14b) show pH independent heights of the waves, as expected for a quaternized form of the polyvinylpyridine. Finally, the electrochemical results offer clear evidence that intergallery polymerization leads to the formation of the 1,6-polyelectrolyte. Figure 14c shows the response for the [Fe(CN)6]3-/4couple at pH 4. For this experiment a clay-modified electrode was immersed for 2 days into a 1 mM solution of 4-VPH+Br- for interlayer polymerization. The corresponding response from a similar experiment conducted at pH 7 is shown in Figure 14c. Again from the pH independence of the two waves we conclude that the polymerization at either pH leads to the quaternized form of the polymer. Figure 14. Cyclic voltammograms (u ) 100 mV s-1) for Fe(CN)3-/4- recorded at pyrolytic graphite electrodes coated with (a) clay-1,2-PVPH+Br-, (b) clay-1,6-PVP+Br-, and (c) clay-4-vinylpyridine monomer. Experiments were conducted at pH 4 (left) (0.1 M acetate buffer) and pH 7 (right) (0.1 M Na2SO4).
Acknowledgment. Helpful discussions with Prof. E. P. Giannelis are gratefully acknowledged. This work was partially supported by the Greek Secretariat of Research and Technology, through the PENED program. CM981140Z
